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Eusperm and parasperm ultrastructure have been studied for the first time in the neogastropod Plicopurpura patula. Eusperm structure is typical of many Neogastropoda, in which an apical bleb characterizes the conical acrosome with subacrosomal granule; the nucleus is tubular and penetrated to its tip by basal body and axoneme; the mid-piece comprises spiral mitochondria that ensheath the axoneme but terminate at the annulus, defining the junction with the end-piece. In the proximal portion of the end-piece dense tracts of glycogen match the axoneme but these disappear distally. The parasperm is unusual in having no axonemes and no motility was observed. Eusperm do not attach to parasperm, as occurs in the similar nurse cells of Littorinimorpha, thus potential functions may be limited to some form of paternity assurance as occurs in some insects. The nucleus of the parasperm is eliminated during paraspermiogenesis, but there is considerable post-transcriptional secretion activity resulting in the production of a variety of granules, some rich in glycoproteins. A prominent paracrystalline rod accumulates in the centre of the parasperm also from Golgi-derived secretions. Preliminary evidence from proteomics suggests that this parasperm carries the heat shock protein HSP70 that could play a role in fertilization inside the female. [ABSTRACT FROM AUTHOR]